Heretofore bulk memories for computer systems have frequently utilized magnetic core technology wherein a large number of magnetic rings are employed with each ring serving as a memory storage element. The data state of the ring is dependent upon the direction of magnetization of the ring. A primary advantage of this type memory is that the information stored in the memory is not lost when power is removed. The rings remain magnetized in the selected states even when power is not supplied to the memory unit. A magnetic core memory can be reactivated and returned to use immediately upon reapplication of electrical power. There is no need to reload the programs and data into memory each time the power is disconnected.
In large computer systems, programs and data are frequently stored on disk memories so that the system can be initialized from disk, even after a power failure. But in smaller computer applications, programs and data are often entered manually and are not stored in any readily accessible manner. Therefore, a power loss which causes loss of the stored bit pattern in memory is a serious failure of the system which cannot be remedied by merely restoring main power.
There have recently come into widespread use semiconductor random access memories which have decided advantages over the older core type memories. In particular, the newer memories are faster, have lower power consumption and occupy less space. However, a serious drawback in the use of semiconductor memories is that each of the memory element circuits is volatile, that is, the information stored in the memory element is lost when power is removed from the memory circuit. With such a memory system the programs and data stored in the memory are lost whenever power is removed from the memory unit. Although the loss of power does not result in circuit damage, the loss of stored information does require that the system be reloaded with programs and data before processing can continue. The reloading of programs is a time consuming process which reduces the efficient utilization of a computer system. In certain types of systems a provision is made to transfer the contents of memory to disk when a failure is first indicated. But in many computer systems a power failure occurs so rapidly that all of the memory contents cannot be transferred to disk. This is particularly true for process control systems.
It has been proposed to solve the data loss problem by using an additional pin terminal on memory type semiconductor circuits and that this terminal be supplied with a backup power supply to maintain the data in the memory cells. But the mere addition of another pin does not fully solve the problem. The backup power supply generally comprises a battery system and as such cannot supply the heavy current requirements of semiconductor memories for any period of time. Further, there are now established standardized pin patterns for most integrated circuit memories and the addition of another pin dedicated to the backup power supply would constitute an inconsistency in the standards and would require substantial redesign in existing circuits.
Therefore, there exists a need for a backup power circuit for semiconductor memories wherein the standard pin configuration is not effected, the data pattern is retained despite a loss of the main power supply, and the circuit is internally powered down so that the power consumption used to retain memory is sufficiently small to be supplied by a backup battery of reasonable size, yet for a long period of time.